Digital (i.e. numeric) audio Equalization (also noted EQ or EQUA) is a widely known functionality needed in many audio applications, such as the enhancement of audio rendering in music player configuration or the correction of audio transducers such as loudspeakers or microphones. Among equalization techniques, graphic audio equalization allows the user to modify the audio equalization curve visually and in real-time. A graphic equalizer generally presents a limited amount of spectral bands and the user of the equalizer is free to modify the amplitude, generally in a decibel (dB) scale, in a limited range for each band (e.g. between −12 dB and +12 dB).
Many audio graphic equalization realizations exist. The complexity of realizations usually depends on the relying technology (whether the signal is analog or digital). A vast literature describes many ways of realizing graphic audio equalizers suited for processing a numeric audio signal. Different designs correspond to different needs.
One category of graphic audio equalizers suited for processing a numeric audio signal relies on multi-rate implementations with a polyphase analysis (respectively synthesis) network. These are known as efficient digital implementations for uniform filter bank. With these equalizers, a tree structured filter bank can implement logarithmic scaled bands. Multi-rate solutions are efficient but quite complex to realize. Thus, multi-rate implementations are not appropriate for low-latency applications.
Another category of graphic audio equalizers suited for processing a numeric audio signal relies on a FIR direct-form implementation with an adhoc FIR coefficient optimization procedure, such as the well known least-square procedure. This design method is well suited for linear-phase filter design cases. However, these equalizers also present a high latency. Furthermore, a FIR direct-form implementation requires a large amount of taps (and therefore a significant complexity) in order to cover low-frequency bands.
A last category of graphic audio equalizers suited for processing a numeric audio signal relies on a bank (i.e. a set) of band-pass filters, tuneable in gain. Such equalizers have an improved latency and a simple architecture.
The filters of the bank may be of constant band-pass. The paper entitled “Using Stereo 10-Band Graphic Equalizer using the DSP56001”, Motorola, 1988 provides an example of a graphic equalizer belonging to this last category with ten bands implemented as a constant band-pass biquadratic filter. In other words, the band-pass, or the quality factor (Q), is not tuneable. Such equalizers present an issue of spectral ripples, notably when similar gains are given in adjacent bands. The paper entitled “Graphic Equalizer design Using High-Order Recursive Filters”, M. Holters, U. Zölzer, Proc of DAFx-06, 2006, and U.S. Pat. No. 5,687,104, discuss ways of limiting such spectral ripples. However, the issue of ripples remains significant.
Other graphic equalizers belonging to this last category, based on variable-Q filters (e.g. band-pass filters with a tuneable, i.e. configurable, band-pass), such as the solution discussed in U.S. Pat. No. 7,266,205 B2 allow a better control of the ripple phenomenon. The implementation of variable-Q filters however presents some issues, notably within the context of fixed-point processors. Variable-Q filters may present ringing audio artefacts when IIR parameters are switched (while the signal is processed). Also, local filter divergence or clipping may occur when IIR parameters are interpolated. Finally, there may be a formation of limit cycles when the system returns to rest. This is notably due to the fact that variable-Q filters have variable parameters that belong to feedback loops.